Today is 7 August and if you go out tonight at sunset you will see a FULL moon rising in the east. It will be overhead at midnight, and set at sunrise. It makes sense, sun, earth and moon are in a straight line, so sun and moon are as far apart in the sky as possible (in degrees). The moon will be in partial (about 25%) eclipse tonight, as it nicks the edge of the earth’s shadow. This eclipse will be visible in the W Pacific, Asia, Antarctica, Africa and most of Europe.

Exactly a fortnight later (21 August), the NEW moon will be on the opposite side of its orbit, between earth and sun, and we will be directly under its shadow. Eclipses tend to come in pairs, one solar and one lunar, for obvious reasons due to the geometry. There aren’t always eclipses at new and full moon because sometimes the three bodies don’t line up precisely. The moon’s orbit is inclined about 5 degrees to the ecliptic (the sun’s path in the sky), and the ecliptic is inclined about 23.5 degrees to our equator. The earth and moon sometimes go above or below each other’s shadow as a result, but sometimes, like this month, the alignment is right on.

I know you can figure that out without my help, but sometimes it helps to point those things out. The geometry of the celestial sphere is not that complicated, but it took us thousands of years as a species to get it straight because from our point of view inside the system it is hard to untangle all the motions. But the funny thing is that we’ve been accurately predicting eclipses for thousands of years, even long ago when we believed the earth was the center of the universe and everything orbited around it. You can tell time, and devise a good calendar without a true knowledge of how the planets move. You can even determine a ship’s position with a sextant and still believe the earth is the center of the solar system.

Its a good thing to keep in mind. You don’t need to know exactly how things work to figure out how to predict the way they’ll turn out. It explains how we sometimes get things right, and still fuck up big time every now and then.